Field of the Invention
The present invention relates to a resin piece sorting method and a resin piece sorting apparatus, and particularly relates to a method of sorting resin pieces for removing the resin pieces containing a specific element, from a large number of resin pieces including various kinds of resins and from foreign materials other than the resin, which have irregular shapes, as well as a resin piece sorting apparatus for which this method is used.
Description of the Background Art
When an object made of a certain substance is irradiated with X-rays, the amount of X-rays absorbed by the object is determined by the kinds and density of elements contained in the object as well as the thickness of the object. In order to differentiate between a plurality of objects made of different materials based on X-ray absorption characteristics, it is necessary to take into consideration the influence of an element contained in an object on X-ray absorption and the influence of the thickness of an object on X-ray absorption. In the case where the former influence is larger than the latter influence, what material makes an object can be determined based on the X-ray transmittance. For example, when a metal foreign material contained in a food is to be identified, whether or not the metal foreign material is contained can be determined merely by acquiring an X-ray transmission image.
In the case however where respective atomic numbers of elements contained respectively in samples are close to each other, the influence of the thickness of an object on X-ray absorption is not negligible, relative to the influence of a contained element on X-ray absorption. It is therefore fundamentally difficult to differentiate, based on the amount of absorbed X-rays, between substances contained respectively in a large number of objects having respective shapes different from each other.
In the case where an object contains substances having respective X-ray absorption tendencies close to each other, identification for example of a bone in a body tissue or a bone foreign material in a food by measuring absorption of X-rays can be done by means of a technique known as energy subtraction method. According to the energy subtraction method, two kinds of X-rays having respective energy regions different from each other are applied to an object, respective characteristics of absorption of these X-rays are measured, and the difference therebetween is calculated to thereby enable highly sensitive detection of a difference between substances contained in the object. Specifically, for an object to be measured, a characteristic of its absorption of a low energy X-ray and a characteristic of its absorption of a high energy X-ray are measured, a natural logarithm of the transmittance of each X-ray is taken, a weight is applied by an appropriately selected parameter, and thereafter the difference is calculated. As will be described later herein, a weighting factor can appropriately be selected so that the differential value for a specific substance in the object is theoretically zero, regardless of the thickness of the object. Accordingly, the specific substance can be discriminated from other substances with high sensitivity.
In order to use the energy subtraction method, it is necessary to measure absorption of two kinds of X-rays having respective energies different from each other. Formerly two X-ray sources and two X-ray sensors were used. Currently an X-ray source generating continuous X-rays and a dual energy X-ray sensor capable of detecting each of absorption of a low energy X-ray and absorption of a high energy X-ray are used. One X-ray source and one dual energy X-ray sensor are used to enable the energy subtraction method to be utilized.
In order to make the most of the effects of the energy subtraction method, it is necessary to set the weighting parameter to an optimum weighting parameter. As a technique for automatically setting the parameter, a technique has been proposed that acquires an image with the intensity of transmitted X-rays and thereafter makes an independent component analysis (see Japanese Patent Laying-Open No. 2010-91483 for example). According to this technique, an image in which only a substance to be identified is enhanced can be obtained through a series of: acquisition of an image; image conversion for the independent component analysis; setting of a parameter from the converted image; and acquisition of an image in which only a foreign material is enhanced.
Meanwhile, the waste plastic recycling business requires a technique of accurately differentiating resin pieces and metal foreign materials to be removed that contain additives which hinder recycling, from resin pieces of miscellaneous waste plastics, and sorting only useful resin pieces appropriate for recycling.